The invention relates to a slide ring seal arrangement for high circumferential velocities for the sealing of liquid fluids with low to no electrical conductivity, such as low-salt boiler feed water in the case of boiler feed-water pumps or the like, having at least one pair of slide rings, of which one can be installed in a rotationally immovable manner and the other installed for common rotation with a rotating component, which slide rings have interacting slide faces which during operation form between them a sealing gap which seals a region of the slide ring seal arrangement which is pressurized by the fluid to be sealed in relation to a surrounding region, wherein one of the slide rings is formed basically from a material with good electrical conductivity and the other slide ring is formed basically from a material with low electrical conductivity, wherein the slide ring seal arrangement is located in a dead-end circulation circuit of a rotating component which is to be sealed and is connected to a cooling system which has a filter.
The primary field of application of the invention concerns boiler feed-water pumps, wherein a boiler feed water which is to be delivered frequently has temperatures of up to 200° C. and is generally in the fully desalinated state. In order to protect the slide ring seals, which are used in such boiler feed-water pumps, against overheating, such sealing systems are provided with a cooling system. Such a slide ring seal system is disclosed in the Burgmann design manual 15.3, slide ring seals, edition of Oct. 13, 2005 on page 129 in the left-hand column 1. The slide ring seal, in accordance with API 682 or ISO 21049, is constructed with a circulation system as a so-called dead-end according to plan 23, cf. also page 97 of the design manual. This circulation system—for a pump shaft which is to be sealed—provides a slide ring seal arrangement which is connected to a separate heat exchanger. For protection of the slide ring seal, two magnetic filters are arranged in a dissipating cooling line in a 2-stranded section, by means of which metallic impurities in the circulation circuit of the slide ring seal are trapped. This sealing system is decoupled from the actual feed-water circuit of the pump, which is why the term dead-end has become established for it. In the circulation circuit, the heat exchanger is connected in a simple manner to an external cooling system so that the slide ring seal has a separate closed lubrication and cooling circuit. In this way, the maintaining of an operating temperature of below 100° C. is made possible for the slide ring seal.
Such slide ring seals are exposed to a delivery medium which has a very low electrical conductivity, being <1 μS/cm. In addition, the slide ring seals are subjected to very high circumferential velocities, frequently above 40 m/sec, as a result of which damage to the slide ring seals occurs after relative short operating times in the event of unfavorable operating parameters.
In such slide ring seal arrangements, which are known, for example, from EP 845,622, having a material combination of silicon carbide and carbon for their slide ring pairing, damage frequently occurs in the region of the silicon carbide ring as a result of the boiler feed water, which is in a desalinated prepared state and also used in the circulation circuit, on account of the high circumferential velocities. For avoiding said damage, it is proposed to discharge the electric charge caused by the high circumferential velocity on the poorly conducting silicon carbide ring from the circumferential region of the rotating slide ring to the surroundings, especially to the pump housing. For this purpose, the stationary slide ring, which is located in the housing, is provided with a conductive coating in order to therefore avoid the forming of a potential in the region of the slide ring seal.
DE 199 46 219 A1 shows a comparable method, in which for improving the service life it is proposed to pick off an electrical potential, which builds up on the slide ring seal during operation, from the shaft of the pump and to discharge it to an earthing point. This is to be carried out by a slip ring system, in which the electrical potential is picked off directly from the rings of the slide ring seal. Such a solution may perhaps function in specific individual cases, but it is contingent upon a direct mechanical engagement in the region of the slide ring seal, as a result of which further malfunctions and leakages can occur.
Furthermore, DE 199 46 219 A1, as prior art, mentions in its descriptive introduction the use of costly dosing devices on the slide ring seals, which introduce alkalizing or oxygen-binding chemicals in the region of the slide ring seal. Such dosing devices are separate from the water and steam cycle of a power plant unit and act only upon the separate circulation circuit of the slide ring seal arranged in the dead-end. These dosing devices, which are constructed as an additional dosing station, provide an alkaline mode of operation for the slide ring seal. The dosing devices have chemical tanks, a plurality of dosing pumps with overflow valves, additional dosing strands with magnetic valves and isolating valves arranged therein, and a plurality of conductivity controllers and a multiplicity of measuring probes for installing in the circuit. Deviations from the permissible operating state are signaled by an alarm. This dosing device feeds ammonia, and on occasion also a binding agent for oxygen in addition, into the circulation circuit. In addition to the complex piping arrangement, this also requires a permanent, complex chemical control of the water state or fluid state in the dead-end. Such dosing devices or dosing stations are associated with high plant-engineering and financial outlays, require a significant monitoring cost and include the risk of malfunction susceptibility.